Is it permissible to fast as expiation for an oath when one does not have the financial means, or must one wait until one acquires the money to pay the expiation?

Swearing oaths is not a legitimate means of disciplining oneself and preventing sin. Frequent oath-taking can put the swearer in a difficult position and may lead to not fulfilling the oath, as Allah Almighty said: "And guard your oaths." Expiation is a way out of sin. It is forbidden to break an oath made to abandon something unlawful. The expiation for an oath becomes immediately obligatory if the reason for breaking it was a sin. One who is unable to pay money must promptly expiate by fasting, and it is not permissible to delay giving the expiation until money becomes available.